1. Field of the Invention
The present invention relates to a magnetic head for use in combination with a cassette having a tape pressure pad for making gliding contact with a tape-gliding face, and more particularly to a magnetic head having a pair of projecting stripe portions for reducing the contact pressure of said tape pressure pad.
2. Related Background Art
Among such magnetic head there is for example known a magnetic head for recording and reproducing audio analog signals in a cassette tape recorder, of a general structure as shown in FIG. 1.
In FIG. 1, each of magnetic cores 12 has a magnetic gap 14 at a front end face thereof, and is provided with an unrepresented coil at an unrepresented rear portion. Said magnetic cores 12 are fitted in a shield case 15, and are fixed by a fixing material 13 such as epoxy resin to constitute a magnetic head 1.
The upper face of the shield case 15 in the drawing constitutes a magnetic tape-gliding face (hereinafter written as gliding face) 10 to be in gliding contact with an unrepresented magnetic tape, and the front end faces of the magnetic cores 12 are exposed, on said gliding face, in an aperture 15a formed in the shield case 15.
Tape guide members 2, 2 for guiding the magnetic tape are fixed on the lateral sides of the shield case 15 at the tape entrance and exit sides of the gliding face 10, and a head mounting plate 30 is fixed to the bottom face of the shield case 15.
The magnetic head 1 of the above-explained structure is placed, at recording and playback in a cassette tape recorder as shown in FIG. 2, in gliding contact with a magnetic tape 3 of a tape cassette 9.
At the recording and playback operation, the gliding face 10 of the magnetic head 1 projects, as shown in FIG. 2, by a projection height H from a linearly extended position of the magnetic tape 3, represented by a broken line. Said projection height H is defined as 1.8.+-.0.5 mm according to the IEC standards.
In the magnetic tape cassette 9, a pad 6 elastically supported by a pad spring 5 is positioned opposite to the magnetic head 1 and resiliently presses the magnetic tape 3, by means of the elastic force of said pad spring 5, toward the gliding face 10 of the magnetic head 1, in order to prevent the spacing loss caused by the gap between the magnetic tape 3 and the magnetic core 12. Such pressure contact state is shown in FIG. 3, representing a cross sectional view along a line a--a' in FIG. 1. Said pad 6 has a width of at least 5.00 mm according to said standards, larger than the width 3.81.sup.+0.sub.-0.05 mm of the magnetic tape 3, and is composed of a rectangular block for example of felt.
Rotation of a capstan 8 and a take-up reel 4 of the recorder as indicated by arrows pulls out the magnetic tape 3 from an unrepresented unwinding reel at the right-hand slide of the tape cassette 9, whereby the tape 3 glides over the magnetic head 1 and is then taken up on the reel 4 as indicated by arrows. A pinch roller 7 is provided for maintaining the magnetic tape 3 in contact with the capstan 8.
During said gliding contact, the magnetic head 1 executes a magnetic recording on or a magnetic playback from the magnetic tape 3.
FIG. 4 shows another conventional structure of the magnetic head 1 disclosed in the Japanese Utility Model Laid-open No. 12704/1975, portions 16a-16d for defining the running position of the magnetic tape are extendedly formed, instead of the tape guide members 2, along the gliding face 10 of the magnetic head 1 in the integral manner.
At the internal rims of the connecting portions of said defining portions 16a-16d, there are formed recesses 17, 18 for accepting the above-mentioned pad 6. Consequently, also in this magnetic head 1, the pressure contact of the magnetic tape to the gliding face 10 by the pad 6 is achieved in the same manner as in FIG. 3.
However, in the above-explained mechanisms, it is increasingly requested to reduce the torque T required for winding the magnetic tape 3 in gliding contact with the magnetic tape 1, in order to decrease the power consumption of the tape driving mechanism.
For this purpose it has been proposed, for example, to include a lubricating material in the fixing material 13 in the head structure shown in FIG. 1 for reducing the friction coefficient of the gliding face 10, but sufficient effect has not been obtained.
FIG. 5 is a chart showing the torque T required for winding the magnetic tape 3 as a function of the projection height H of the magnetic head 1, when it is in gliding contact with the magnetic tape 3 of the cassette 9 as shown in FIG. 2. A line 19A indicates the behavior under pressure contact with the pad 6, while a line 19B shows the behavior without said pad. The measurement was conducted with a torque gage in an atmosphere of temperature of 25.degree. C. and relative humidity of 50%.
As will be apparent from FIG. 5, the torque T is proportional to the head projection H as indicated by the line 19A in the presence of pressure contact by the pad 6, but is almost constant regardless of the head projection H in the absence of the pad 6, as shown by the line 19B.
Since the contact pressure of the magnetic tape 3 to the magnetic head 1 by the pad 6 increases with the projection height H of the head 1, the torque is apparently dependent on the contact pressure of the pad 6 and becomes larger as said pressure increases.
Consequently, the torque T of the magnetic tape 3 can be decreased by reducing the contact pressure of the pad 6.
In relation to this fact, the U.S. Pat. No. 3,777,070 and the Japanese Utility Models Laid-open No. 21022/1974, 36922/1974, 131919/1974 and 7422/1975 proposed, as shown in FIG. 6, a structure in which a pair of projecting portions 20, 20 are formed on the gliding face 10 of the magnetic head 1, on both sides of the gliding area of the magnetic tape 3, whereby the pad 6 engages at both lateral ends thereof with said projecting portions 20, 20 and is not pressurized to but separated from the magnetic tape 6. The U.S. Pat. No. 3,777,070 disclosed such structure for a magnetic head for recording and reproducing digital signals, while the Japanese Utility Model Laid-open No. 36922/1974 disclosed a structure for a Hall element.
However, such structure in which the magnetic tape 3 is not under pressure contact, gives rise to a spacing loss though it can reduce the torque. The spacing loss is not a serious problem in a magnetic head for digital signals or a Hall element, but is an important drawback for the magnetic head for recording and playback of audio analog signals, particularly in the faithful recording and playback of delicate changes in the audio wave forms. This will be referred to as a first drawback.
Then, as explained in relation to FIG. 2, the magnetic tape 3 runs in the cassette 9 with gliding contact with the gliding face 10 of the magnetic head 1.
In such structure, therefore, the adhesive material of the magnetic tape 3, such as binder, sticks to the gliding face 10 of the magnetic head 1 in the course of gliding movement of the tape on the head 1, thereby inducing a loss in the head output. Said adhesive material at first adheres to the fixing material 13 of the gliding face 10, and then spreads to the area of magnetic cores 12, thus inducing the loss in the head output. Particularly under a condition of high temperature and high humidity, for example encountered by a magnetic head for automobile use, the deposition of the adhesive material is accelerated, causing an output loss of 3 dB or more for example by tape running as short as about 10 hours.
It has therefore been proposed, as shown in FIG. 7, to form grooves 28, 28 along both sides of the magnetic cores 22 at the entrance and exit sides of the gliding face 26, thereby removing the fixing material 24 in the positions of said grooves.
However, such structure, though effective for preventing the deposition of the adhesive material, increases the abrasion of the magnetic cores 22 as the pressure of the pad 6, for pressing the magnetic tape against the gliding face 26, is concentrated on the magnetic cores 22. For example the abrasion amounts to 30 .mu.m or more in a tape running of about 200 hours. Such abrasion gives rise to a spacing loss, indicing a loss in the heat output.
FIG. 8 is a chart showing the abrasion of the magnetic core 22 as a function of the head projection height H in the tape gliding state shown in FIG. 2, indicating that the magnitude of abrasion is dependent on the pad pressure. The amount of abrasion was measured with a surface coarseness meter after a tape running of 200 hours under 40.degree. C. and 70% RH.
As shown in FIG. 8, the abrasion rapidly increases with the increase of the head projection H beyond 1.0 mm. This result indicates a fact that the abrasion is dependent on the pad pressure, as said pressure increases at a large head projection H. This point will be referred to as a second drawback.
In a recording-playback apparatus such as a cassette tape recorder, a tape driving system as shown in FIG. 9 is employed for maintaining the magnetic tape in gliding contact with the magnetic head.
In FIG. 9, a magnetic head 1 is mounted, by a mounting plate 30, on an unrepresented main body of the cassette tape recorder. The tape cassette 9 shown in FIG. 2 is loaded on this tape driving system. In FIGS. 2 and 9, there is shown a capstan 8 for driving the magnetic tape 3 at a constant speed. Said capstan 8 is fixed to a socket 32 integral with a flywheel 31 which is driven by a motor 34 through a belt 34, and a uniform rotating speed is obtained by the inertia of said flywheel 31.
A rubber pinch roller 7 maintains the magnetic tape 3 in pressure contact with the capstan 8.
At the recording or playback operation, the magnetic head 1 as shown in FIG. 2 is advanced by a predetermined amount into the interior of the tape cassette 9, thereby pressing the pad 6 across the magnetic tape 3. The pad spring 5 exhibits elastic deformation, and the resulting elastic force causes the pad 6 to press the magnetic tape 3 to the gliding face 10 of the magnetic head 1. Since the surface of the pad 6 is formed parallel to the gliding face 10, the pressure of the pad 6 is substantially uniform over the surface thereof.
Rotation of the capstan 8 indicated by an arrow, caused by the motor 33, in contact with the pinch roller 7 with the magnetic tape 3 sandwiched therebetween, advances the magnetic tape 3 as indicated by an arrow with gliding contact with the gliding face 10 of the magnetic head 1, thus achieving a recording or blackback operation. Simultaneously the take-up reel 4 is rotated by unrepresented driving means to wind the magnetic tape 3.
In such magnetic head for a cassette tape recorder, in order to reduce the torque required for the magnetic tape 3 and the frictional abrasion of the magnetic head, it is already proposed to form a pair of projecting stripe portions on the gliding face 10, for contacting the end portions of the pad 6 exceeding the sides of the magnetic tape, thereby reducing the contact pressure of the pad 6.
In the above-explained structure, the sandwiching pressure on the magnetic tape 3 between the capstan 8 and the pinch roller 7 is not transversally uniform but is stronger toward the socket 32, since the capstan 8 is supported at one side only. Consequently the traction force on the magnetic tape 3 is stronger at the side closer to the socket 32 in the transversal direction, and the tension of the magnetic tape 3 shows a similar distribution.
The abrasive force of the magnetic tape 3 on the gliding face 10 of the magnetic head 1 is represented by the sum of the contact pressure of the pad 6 and the tension of the tape 3. Therefore, even if the contact pressure of the pad 6 is uniform as explained above, the gliding face 10 causes one-sided abrasion if the tension of the tape is not uniform. In fact the abrasion of the gliding face 10 becomes larger at the side of the socket 32. Such uneven abrasion increases the spacing loss, thus eventually shortening the service life of the magnetic head 1, and this fact will be referred to as a third drawback.